The field of this invention relates to a method of manufacturing integrated circuit devices, and more particularly, to a technique of manufacturing buried contacts in recessed gate MOSFET semiconductor devices.
As used herein, the term "recessed gate" MOS device refers to a field effect transistor having a thick oxide layer over its source and drain regions. This is a more expansive use of the term than that by V. L. Rideout and D. J. Silvestri in their paper, "MOSFET's With Polysilicon Gates Self-Aligned to the Field Isolation and to the Source and Drain Regions," IEEE Transactions on Electron Devices, Vol. ED-26, No. 7, July 1979, pp. 1047-1052, and the reference cited therein. In Rideout and Silvestri paper the term refers to a particular MOSFET device structure in which the silicon gate is doubly self-aligned with respect to the field regions and to the source and drain regions of the device.
The present invention, on the other hand, is directed toward a problem related to any MOS transistor in which a thick oxide, like that in the field regions of the integrated circuit, is used to also cover the source and drain regions of the transistor.
What is common to the recessed gate technology is a problem of making electrical contact to the conducting regions in the substrate, normally defined by the source/drain mask in process. This is the problem of the buried contact.
A straightforward solution is to place the contact apertures within the source/drain regions of the integrated circuit as viewed from above. However, to ensure that the contact mask defines contact regions entirely within the source/drain regions, the source/drain regions must be widened. This will protect against the expected variations in mask alignments during processing. But the widening of the source/drain regions lessens the maximum packing density of the integrated circuit for a given photolithographic technology.
A second drawback to this solution is that in the recessed gate technology, a thick thermal oxide at several thousands Angstroms is grown over the source/drain regions. Etching consistent and accurate contact apertures as defined by the contact mask through such thick oxide layers is difficult.
Other solutions to this problem have also various shortcomings for reasons discussed below. The present invention, by its novel approach, avoids these shortcomings and is a significant advance over the prior art.